A high fuel-value coal-water slurry which can be injected directly into a furnace as a combustible fuel can replace large quantities of fuel oil presently being used by utilities, factories, ships and other commercial enterprises. It is highly desirable that the production time and expense of such coal-water fuel slurries be minimized as much as possible.
For efficient practical use as a fuel, the coal-water slurry must have several essential characteristics. It must have long-term static stability so that it can be stored for extended periods of time by suppliers or at the point of use. During such storage, the slurry must remain uniformly dispersed or, at most, be subject to some soft subsidence which can be easily redispersed by stirring. Uniform dispersion is essential for reliably constant heat output. Coal loadings must be sufficiently high; for example, 60 to 70 percent or higher, to produce adequate fuel value despite the presence of an inert water carrier. The slurry must also be sufficiently fluid; that is, have a sufficiently low viscosity, to be pumped to and sprayed into a combustion chamber.
Generally, the prior art has focused on reducing the viscosity of a coal-water slurry by the use of anionic ammonium, alkali metal, or alkaline earth metal organic dispersants. The anion of such dispersants is a high molecular weight organic moiety which attaches to the coal particles to give them a high negative charge or zeta potential, which in turn causes repulsion sufficient to overcome Van der Waal's attraction, thereby preventing flocculation which leads to an increase in viscosity. U.S. Pat. No. 4,526,584 to Funk, for example, suggests the use of a dispersing agent to promote deflocculation of carbonaceous particles, preferably in the presence of advantageous electrolytes. Funk teaches that the presence of monovalent cations such as Na.sup.+, or Li.sup.+ or K.sup.+ tends to promote deflocculation, and further advises against the use of multivalent cations such as Ca.sup.2+, Al.sup.3+ and Mg.sup.2+ which tend to cause the carbonaceous particles to flocculate under certain conditions. Funk also teaches that it is preferred to incorporate an advantageous monovalent electrolyte, such as an ammonium or alkali metal base, into an aqueous slurry to increase deflocculation of the slurry. U.S. Pat. No. 4,504,277 to Scheffee suggests the use of a combination of an ammonium salt organic dispersant and an alkaline earth metal salt organic dispersant for improving the viscosity and storage stability of a coal-water fuel slurry. None of the references teaches or suggests the unique capability of soluble inorganic alkaline earth metal salts to control the stability and viscosity of a coal-water fuel slurry. None of the references teaches or suggests controlling the concentration of polyvalent cations in a coal-water fuel slurry in order to control the viscosity and stability of the slurry.